AZ31B vs AZ91D Magnesium Alloys: Composition, Product Form, Processing and RFQ Guide

AZ31B and AZ91D are both magnesium-aluminum-zinc alloys, but they are not two interchangeable plate grades. AZ31B is primarily specified as a wrought alloy for sheet, plate, extrusion, bar or forging routes. AZ91D is primarily a high-purity casting alloy, especially for die castings. The product form and manufacturing route are therefore the first selection decision, before comparing strength, corrosion or machinability.

This guide helps design engineers and buyers compare AZ31B with AZ91D, interpret the alloy names, select the correct ASTM product standard and prepare a drawing-based RFQ. It does not certify a substitution, flammability control, aerospace approval, welding procedure, corrosion life or mechanical property. Final alloy, temper, casting condition, dimensions, surface protection, inspection and safety controls must follow the customer-approved design and current standard.

The alloy designation is a chemistry code, not a product specification

Under the ASTM magnesium designation system, the first letters identify the principal alloying elements and the numbers indicate their approximate nominal percentages. A represents aluminum and Z represents zinc, so AZ31 broadly indicates about 3% aluminum and 1% zinc, while AZ91 broadly indicates about 9% aluminum and 1% zinc. The final letter distinguishes registered composition variants; it is not a temper.

That shorthand cannot replace controlled chemistry limits. The ASTM standard, product form and edition govern acceptance. Residual iron, nickel and copper are particularly important in magnesium corrosion control, and the relevant limits differ by alloy and specification.

Designation layer	AZ31B	AZ91D
Nominal chemistry concept	Mg-Al-Zn with roughly 3% Al and 1% Zn	Mg-Al-Zn with roughly 9% Al and 1% Zn
Primary product family	Wrought sheet, plate, extrusion, bar and forgings	Cast product, commonly high-pressure die casting
Key ASTM example	ASTM B90/B90M for AZ31B sheet and plate	ASTM B94 for magnesium-alloy die castings
Letter B or D	Registered alloy variant	Registered alloy variant
Not established by the name	Temper, process, properties, dimensions, corrosion finish and component acceptance

Do not order AZ91D plate as though it were ASTM B90 AZ31B plate

ASTM B90/B90M-21 covers AZ31B magnesium-alloy sheet and plate in O, H24 and H26 tempers. ASTM B94-18, reapproved in 2025, covers magnesium-alloy die castings including AZ91D. These standards address different manufacturing routes, test expectations and product geometry. A machined slab, cast plate, die casting and rolled plate can share approximate external dimensions while having very different microstructure, properties and discontinuity patterns.

If a drawing calls for AZ91D plate, ask what functional requirement produced that callout. The designer may actually require a die-cast housing, a machined casting or simply lightweight magnesium stock. Do not silently substitute AZ31B, and do not promise wrought properties for cast AZ91D. Resolve the intended product route in writing.

Composition differences and metallurgical effect

Element	General role in AZ31B/AZ91D	Engineering caution
Magnesium	Base metal providing low density	Low density does not establish stiffness, strength or fire risk of a finished design.
Aluminum	Contributes strength, casting response and phase balance	Higher Al in AZ91D does not make every property better or create a wrought plate.
Zinc	Supports strengthening within controlled limits	Interpret with Al, condition and process.
Manganese	Helps manage harmful iron effects in high-purity magnesium systems	ASTM B94 includes Fe/Mn relationship controls for AZ91D.
Fe, Ni and Cu	Controlled impurities important to corrosion behavior	Small deviations can matter; request full heat/lot analysis.
Other residuals	Limited by the applicable standard	A supplier’s typical table is not the acceptance limit.

Luxfer MEL’s AZ31B information specifically distinguishes composition compliance from guaranteed wrought mechanical properties when supplying cast machining stock. That is a useful procurement lesson: chemistry alone does not transfer the properties of one manufacturing route to another.

Microstructure: wrought deformation versus solidification

AZ31B sheet and plate receive thermomechanical processing. Rolling, extrusion, forging, annealing and cold work influence grain structure, texture, anisotropy and temper. Properties can vary with direction and thickness. Bend behavior must be evaluated relative to rolling direction, temper, radius and temperature.

AZ91D die castings solidify rapidly in a metal die. Wall thickness, gate location, flow fronts, intensification, venting, vacuum, die temperature and local solidification affect porosity and mechanical behavior. A separately cast coupon may not represent a thick boss, junction or last-filled region. The drawing and inspection plan must recognize the actual process.

Mechanical properties require condition and test basis

Property question	AZ31B route	AZ91D route
Strength and elongation	Specify temper, thickness, direction and ASTM B90/B90M requirements	Specify casting standard, process, sample basis and component region
Hardness	Useful for process screening when method/location are defined	Can show local variation but does not prove porosity or integrity
Fatigue	Surface, texture, notches, joints and corrosion dominate design	Porosity, surface and section transitions are major variables
Creep/elevated temperature	Neither grade should be approved from room-temperature data; review time-temperature-load duty.
Design allowables	Use purchaser-approved, statistically valid data for the exact product condition.

Do not copy one “typical property” table into a purchase order. Typical values are useful for screening but are not minimum guarantees unless incorporated through the governing standard and contract. Aerospace, pressure, transportation and safety-critical applications require their own approved material and design systems.

Selection by product geometry and production volume

AZ31B plate or sheet is appropriate when the design begins with flat or wrought stock that will be cut, formed, machined or joined. It supports prototypes and lower-volume parts without a dedicated die, although material utilization and machining time may be important. AZ91D die casting becomes relevant for complex thin-wall shapes, integrated ribs and bosses, and production volumes that justify tooling and process qualification.

Do not choose die casting only because a part is complex. Evaluate tooling, draft, ejector location, gate removal, porosity-sensitive machining, leak tightness, dimensional capability and design changes. Conversely, machining every feature from AZ31B plate can create cost, distortion and chip-safety issues. The correct route follows annual volume, geometry, CTQs, load path and qualification requirements.

Forming AZ31B sheet and plate

Magnesium’s hexagonal crystal structure limits room-temperature formability compared with many aluminum or low-carbon steel products. Forming response depends on temper, thickness, grain direction, bend radius, strain rate, tooling and approved heating practice. An annealed O temper and a cold-worked H temper cannot be assumed to use the same radius.

Heating magnesium stock or tooling introduces burn, oxidation and fire-control requirements. Use a qualified forming procedure and site emergency plan; do not improvise with an open flame. If warm forming is required, the responsible engineer should define temperature monitoring, transfer time, lubrication, surface protection and post-form inspection.

Machining and magnesium chip safety

Both alloys can be machined, but fine chips and dust require strict control. Separate magnesium chips from steel, avoid accumulation, use suitable collection and housekeeping, and follow the facility’s approved fire-prevention procedure. Ordinary water or the wrong extinguisher can intensify some metal-fire conditions. Cutting fluid, tool condition, speed, chip form and dust extraction must be established by competent manufacturing and EHS personnel.

For AZ91D castings, machining may open subsurface porosity, especially near thick sections or flow junctions. Define machined sealing faces, stock allowance, leak requirements and allowed impregnation or repair before tooling release. For AZ31B plate, monitor residual-stress distortion, flatness and grain direction.

Joining and welding boundary

Welding procedure qualification must address alloy, temper, joint, filler, shielding, cleanliness, heat input, distortion, porosity and fire safety. A generic statement that “magnesium is weldable” is not an approval for AZ31B sheet or AZ91D die castings. Die-cast porosity and entrapped contaminants can make fusion welding difficult and variable.

Mechanical fastening and adhesive bonding also require design review. Consider galvanic couples, bearing stress, thread inserts, surface treatment, joint stiffness, sealant and service temperature. Isolate dissimilar metals using a validated system where corrosion risk exists.

Corrosion protection and galvanic design

Magnesium is electrochemically active. Environment, impurity control, surface condition, coating damage, crevices and contact with more noble metals influence corrosion. AZ91D’s high-purity controls are important, but the grade name is not a corrosion-life guarantee. AZ31B similarly requires service-specific protection.

Corrosion input	Information to specify	Design response to validate
Atmosphere or fluid	Humidity, salts, pH, temperature, wet/dry cycle and contaminants	Alloy, pretreatment, coating and maintenance.
Dissimilar metals	Fastener/contact material, area ratio and electrolyte exposure	Isolation, sealant, drainage and coating continuity.
Machined edges	Final exposed surfaces and handling	Post-machining conversion/coating repair.
Coating damage	Assembly and service abrasion	Inspection, repair procedure and sacrificial strategy.
Storage	Humidity, packaging and duration	Dry protection without trapping corrosive contamination.

Inspection plan for AZ31B wrought stock

State ASTM B90/B90M edition, alloy, temper, dimensions, flatness, edge, surface and required mechanical tests. Identify the sampling direction and lot definition. If ultrasonic or penetrant inspection is required, specify the method, coverage, reference standard and acceptance criteria. Do not request “UT pass” without an agreed class and calibration basis.

Receiving inspection should connect each plate or bundle to the certificate. Verify marking before cutting, photograph transport damage and maintain heat/lot traceability through nesting and machining. Positive material identification can help screen mix-ups only when a qualified method is suitable for light magnesium alloys.

Inspection plan for AZ91D die castings

ASTM B94 requires chemistry and uniform quality, with soundness requirements agreed between producer and purchaser. A useful casting quality plan defines CTQ dimensions, visual standards, porosity-sensitive areas, radiographic or CT coverage where justified, leak testing, machining validation, sample plan and traceability. Acceptance must reflect function rather than demanding unrealistic zero porosity everywhere.

Use process evidence such as first-article dimensions, shot parameters, die maintenance and capability results where contractually required. Destructive sectioning or density samples can support qualification, but they must be correlated with the actual component region. Any impregnation, welding or repair requires prior written approval and documented reinspection.

Common substitution errors

• Calling AZ31B and AZ91D equivalent because both contain aluminum and zinc.
• Ordering “AZ91D plate” without defining whether it is die cast, gravity cast or machined cast stock.
• Applying rolled AZ31B tensile values to cast stock that only meets chemistry.
• Using a typical supplier composition as contractual ASTM limits.
• Ignoring temper, test direction, wall thickness or porosity-sensitive machining.
• Approving outdoor or salt service without galvanic and coating design.
• Machining magnesium without chip, dust and metal-fire procedures.
• Promising a process or property before the drawing and product standard are agreed.

First-article and change-control plan

A first article should verify more than dimensions. For AZ31B, connect the production lot, temper, rolling direction, forming route, machining sequence, surface preparation and coating to the inspected part. For AZ91D, connect the alloy heat or melt controls, die revision, machine, cavity, shot process, trimming, heat treatment if any, machining and coating. Record the measurement method for every critical characteristic so repeat production is compared on the same basis.

Changes that require purchaser review

Define which changes require notification or requalification: material source, standard edition, temper, plate thickness route, die cavity, gate or vent, casting machine, impregnation, weld repair, machining location, coating chemistry or inspection method. A supplier may consider a change routine while it alters porosity, corrosion or fatigue behavior. The purchase order should state the approval level instead of relying on an informal expectation.

RFQ checklist for AZ31B or AZ91D parts

1. Component drawing, revision, application and annual volume.
2. Required alloy, product form, ASTM standard edition and condition/temper.
3. For AZ31B: sheet/plate thickness, direction, flatness, surface and forming route.
4. For AZ91D: die-casting process, wall map, draft, gates, ejectors and tooling ownership.
5. Mechanical loads, temperature, fatigue, impact and safety classification.
6. Machining, leak-tight surfaces, joining, inserts and dimensional CTQs.
7. Environment, dissimilar metals, coating system and salt/humidity requirement.
8. Chemistry, tensile/hardness, dimensional, NDT/leak and coating records.
9. Repair/impregnation limits, traceability, marking and first-article approval.
10. Quantity, qualification lot, packaging, destination and target schedule.

Related products, quality and factory pages

For wrought-stock enquiries, review AZ31B magnesium alloy sheets and ASTM B90 magnesium sheet. For an existing AZ91D commercial page, see AZ91D magnesium alloy plate suppliers, then confirm the actual manufacturing route. The quality assurance and factory capability pages provide general context.

Send drawings and service conditions for review

Use the contact page to send the drawing, alloy/product standard, temper or casting condition, annual volume, load and environment, coating, inspection, quantity and destination. Final alloy and route must follow the approved drawing and purchase order.

Engineering and safety boundary: This comparison is general procurement guidance. It does not establish design allowables, corrosion life, metal-fire controls, weld qualification, regulatory approval or interchangeability. Qualified customer engineering and EHS personnel retain responsibility for the application.

References

• ASTM B90/B90M-21, Magnesium-Alloy Sheet and Plate.
• ASTM B94-18(2025), Magnesium-Alloy Die Castings.
• ASTM B93/B93M, Magnesium Alloys in Ingot Form for Castings.
• Luxfer MEL Technologies, AZ31B cast billet and product-form limitations.